The teachings with respect to the deflection compensating assembly of the present invention are applicable to any deflection sensitive fabricating machine tool. For purposes of clarity and for purposes of an exemplary showing, the invention will be described in its application to a hydraulically actuated press brake. This is a matter of convenience only, and does not constitute a limitation on the present invention. For example, the teachings of the present invention can be applied to other fabricating machine tools such as shears, bending rolls, presses and the like. The fabricating machine tools can be mechanically or hydraulically actuated.
In its most usual form, a hydraulically actuated press brake comprises a stationary bed, a ram mounted above the bed and capable of reciprocating movement toward and away from the bed in a vertical direction, a pair of substantially identical main cylinders operatively connected to the ends of the ram to shift the ram toward and away from the bed, and a source of hydraulic fluid under pressure to actuate the main cylinders. Deflections of the press brake bed and ram, when under load against a workpiece, constitute a problem which has plagued the industry from its beginning to the present day. Such deflections of the press brake bed and ram cause their cooperating tool or die-carrying edges to go out of parallelism during the work stroke.
The nature of this problem can most easily be explained by selecting, for example, one of the most common forms of press brake work, i.e., the air bending of a flat piece of metal into an angle. In the most usual instance of such an operation, the top or male die is mounted on the lower edge of the ram. When the ram moves downwardly, the top or male die pushes the workpiece into a lower or female die, mounted on the upper edge of the stationary bed. The inherent problem of bed and ram deflection, occurring as the result of bending load, creates a variation in the bend angle along the length of the bend in the workpiece.
Prior art workers have made numerous attempts to overcome this problem. A common practice is to shim one of the dies (usually the female die), or the filler block thereunder, so that the dies are parallel in the loaded condition. This solution is effective, but less than ideal. It is a trial-and-error process and hence very time consuming. Furthermore, it changes for each load condition. In today's factory, the trend to small lot sizes, makes this solution unacceptable.
Another approach has been to crown the tool or die carrying edges of the bed and ram, so that these edges are intentionally not parallel in the unloaded condition. They are closer together in the center than at the ends. The amount of this crown is carefully controlled so that the bed and ram are deflected into a parallel position when subjected to a load equal to some predetermined fraction (say 2/3) of the machine's capacity distributed uniformly over the nominal length of the bed and ram. In bending operations wherein the load is equal to (or substantially equal to) this predetermined fraction of the machine's capacity, the male die penetration into the female die is constant and the bend angle is uniform along its length. For bending operations which depart from these predetermined conditions, this solution does not work well.
U.S. Pat. No. 3,702,558 teaches the provision of a compressible polyurethane member interposed between the lower die and bed of a press brake. The polyurethane member is of substantially the same length and in registry with the bend line of the workpiece. The polyurethane member is of uniform spring constant per linear inch of lower die holder. The polyurethane member exerts upward pressure against the center of the lower die, arching the lower die upwardly at the center toward the upper die to a degree which closely matches the upward arch of the upper die due to deflection. This results in bends in the workpiece of good uniformity throughout the length of the workpiece. However, the teachings of this patent are applicable only to the use of bottoming dies, and air bending of a flat piece of metal into a uniform angle cannot be accomplished with the compensating device of this patent.
In commonly owned, copending application Ser. No. 551,165, filed Nov. 14, 1983 in the name of Raymond J. Graf and entitled DEFLECTION COMPENSATING ASSEMBLY FOR A PRESS BRAKE, there is taught a deflection compensating cylinder and auxiliary cross-member assembly for use on hydraulically actuated press brakes. According to this copending application, a pair of auxiliary cross-members are located to each side of the bed, extending the majority of the length thereof, and being affixed to the bed by a pair of pins, each located near an end of the cross-members and extending therethrough and through the bed. The cross-members have corresponding, coextensive, longitudinal slots formed therein and located centrally thereof. The bed is provided with a similar slot, slightly offset in a vertical direction with respect to the cross-member slots. A deflection compensating cylinder is mounted in the cross-member and bed slots with the cylinder contacting one of the bed and the cross-members, and its piston contacting the other of the bed and the cross members. According to this reference, the compensating cylinder has a working area four times that of one of the main cylinders and is connected to the same hydraulic source as the main cylinders, being subject to the same pressure. As a result, when the bed and ram are under load against a workpiece, the compensating cylinder will remove the downward deflection of the bed and impart to the bed an upward deflection substantially similar to that of the ram. This causes the die-supporting edges of the ram and bed to remain substantially parallel under load. The reference further teaches that the deflection compensating assembly could be mounted on the ram, instead of the bed, to accomplish equivalent results. While the compensating assembly of the copending application can be employed in conjunction with both air bending dies and bottoming dies, as a practical matter the teachings of this copending application are essentially limited to the use of hydraulically actuated fabricating machine tools
The present invention is based upon the provision of an elastic unit or spring unit having a variable spring rate. The deflection of the unit when under a uniformly distributed load and when added to the combined bed and ram deflection, yields a total deflection which is constant along the length of the ram and bed. The elastic unit is located between the lower die and the bed of the press brake, or between the upper die and the ram of the press brake. When the elastic unit is mounted on the bed, the bed and its die will achieve the configuration of the ram and its die when under a uniformly distributed load. When the elastic unit is affixed between the ram and the upper die, then the ram and the upper die (when under a uniformly distributed load) will achieve the configuration of the bed and its bottom die. Since the above noted total deflection is constant, the dies will remain parallel under load, yielding a uniform bend angle in the workpiece. Since the deflections of the ram, the bed and the compensating device are all proportional to the load, the device automatically compensates for a load of any appropriate magnitude.
The compensating assembly of the present invention is relatively inexpensive to manufacture and is more accurate than prior art devices for the length of bend for which it is designed. The compensating assembly of the present invention can be used in conjunction with both air bending dies and bottoming dies. In addition, the compensating assembly can be applied to both hydraulically actuated and mechanically actuated press brakes, and can also be applied to existing press brakes or the like of both the hydraulically actuated and mechanically actuated types.